1. Field of the Invention
The present invention is broadly concerned with improved dryer apparatus and drying methods which maximize dryer efficiency and product exist moisture control, notwithstanding the occurrence of upset conditions such as differences in input air temperature and/or humidity, or the moisture content of incoming product to be dried. More particularly, the invention is concerned with such methods and apparatus wherein the adiabatic saturation ratio (ASR) and the temperature of the output air stream from the dryer are maintained at predetermined, substantially constant levels during drying; such ASR and output air temperature maintenance involves determination of the temperature and humidity of the output air stream and adjustment of recycle and exhaust portions of the output air stream and energy input to the dryer, to maintain the ASR and output air stream temperature.
2. Description of the Prior Art
A variety of continuous dryers have been proposed in the past for drying of agricultural products or processed pellets (e.g., feed pellets). Such dryers include rotary drum dryers, single or multiple-stage conveyor dryers, and staged, vertical, cascade-type dryers. In all such dryers, an initially wet product is contacted with an incoming heated air stream in order to reduce the moisture level of the product; as a consequence, the dryers emit a cooled, moisture-laden output air stream.
Regardless of the type of dryer selected for a particular application, operators are always interested in maximizing drying efficiency, i.e., obtaining the maximum drying effect per pound of fuel consumed. A variety of control systems have been suggested in the past for this purpose. See, e.g., U.S. Pat. Nos. 1,564,566, 2,448,144, 4,513,759, 5,950,325, 5,347,727 and 6,085,443; Zagorzycki, Automatic Humidity Control of Dryers; Chemical Engineering Progress, April, 1983, and Miller, Drying as a Unit Operation in the Processing of Ready-to-Eat Breakfast Cereals:I. Basic Principles and Drying as a Unit Operation in the
Processing of Ready-to-Eat Breakfast Cereals:II. Selecting a Dryer; Cereal Foods World, 33:267-277 (1988). However, the problem of maintaining maximum dryer efficiency while controlling product exit moisture, during the course of a dryer run, which commonly may experience upsets, has not heretofore been satisfactorily resolved.
A known drying parameter is the adiabatic saturation ratio of an air stream, typically the exhaust air stream from a dryer. The ASR is the ratio of air moisture in a given air stream, divided by the saturated air moisture at the same enthalpy. It is usually expressed as a percent, even though referred to as a ratio. An equivalent definition of ASR is the degree of saturation of an air stream when holding enthalpy constant. The humidity ratio for the air stream is divided by the humidity ratio at the intersection of the total enthalpy curve with the saturation curve, using appropriate psychrometric data.
The present invention overcomes the problems outlined above and provides greatly improved drying methods and apparatus which are capable of maintaining high dryer efficiency notwithstanding the occurrence of upsets. Broadly speaking, the drying methods of the invention involve provision of a stream of input air having initial temperature and humidity levels, heating such input air stream to a desired temperature and contacting the heated air stream with an initially wet product in a drying zone to give a dried product and an output air stream. Control of the process is obtained by determining the temperature and humidity of the output air stream on a continuous basis, and using such information to maintain the adiabatic saturation ratio and the temperature of the output air stream at predetermined, substantially constant levels during the drying process, notwithstanding changes in one or more dryer parameters such as input air temperature and/or humidity levels, initially wet product moisture level and combinations thereof. In practice, maintenance of the adiabatic saturation ratio involves recycling a first portion of the output air stream back to the input air stream for mixing therewith, and exhausting a second portion of the output air stream to the atmosphere, in response to the determination of output air stream temperature and humidity. Additionally, the control typically involves adjusting the energy input to the dryer; in most cases, such energy input adjustment includes regulation of the temperature of the heated input air stream, but other energy inputs to the dryer, if any, may also be regulated.
The invention is applicable to virtually all types of convection dryers where a wet product and a heated air stream are contacted for drying purposes. This includes but is not limited to rotary, conveyor, cascade-type, fluid bed and counterflow dryers. To this end, the dryers may incorporate indirect or direct heating of the input air stream; in the latter case, the effects of direct combustion must of course be taken into consideration.
In preferred practice, the dryer is equipped with an exhaust fan/damper unit which serves to draw output air from the drying zone. The control apparatus is coupled with the damper so as to continually adjust as necessary the relative proportions of the output air stream which are recycled and exhausted to the atmosphere. Alternately, in lieu of an exhaust fan/damper unit, a variable speed exhaust fan can be employed. Conventional programmable logic controllers are used in such preferred systems to regulate dryer operation so as to maintain substantially constant ASR and output air stream temperatures.